A standard two-stage air-breathing drive for a rocket comprises a front combustion chamber in which fuel is burnt to produce combustible gases that exit this chamber via a small-diameter nozzle. A diffuser captures air and mixes it with the gases exiting the nozzle in a second combustion chamber where more fuel is burnt, creating substantial expansion that produces thrust out the rear end of the rocket. The fuel used is usually solid and lines the combustion chambers.
In a typical such system as described in German patent document No. 3,104,680 filed by S. Notake et al with a claim to a Japanese priority date of Feb. 12, 1980, the nozzle is provided with an injector in the rear combustion chamber. This injector has radially through going holes aligned with radially inwardly open vents in the cowling of the rocket so that air trapped by the cowling is forced radially into the injector to mix with the gases exiting the front combustion chamber via the nozzle.
In such an arrangement the mixing of the aspirated air with the gases from the front chamber is poor, as the gases emitted by the nozzle are moving at high ultrasonic speeds. Thus such an arrangement is not efficient when using a metal-containing solid fuel, in particular one containing aluminum, as the stoichiometry is incorrect. In this system the mass-flow ratio of air (from outside the rocket) to gas (generated in the front chamber) is therefore about 30:1, or about eight times what the stoichiometry of the reaction would require, so that there is a substantial possibility of the flame going out when such metallic additives are used. At best the efficiency of the combustion is low.
Furthermore when such a nozzle is used it is inadvisable to use a fuel containing a metallic additive, as the metallic particles can condense and plate out in the nozzle. This can result in plugging of the nozzle and complete failure of the jet drive. Furthermore, deflecting the axially arriving additive air radially into the injector means that some of the energy of this air stream is lost.